1. Field of the Invention
The present invention relates to a method and apparatus for preventing degradation of a light source employed in an exposing apparatus such as a graphic arts contact printing machine, a graphic arts copy camera and so on. More specifically, the present invention relates to a method and apparatus for preventing degradation of an exposing apparatus employing a halogen lamp as a light source.
2. Description of the Related Art
A halogen lamp is a type of incandescent lamp. A single substance or a compound of a halogen element such as iodine, bromine, chlorine or the like is sealed in a bulb of the halogen lamp. The halogen lamp comprises a bulb 2 and a filament 3 formed of, for example, tungsten W provided in the bulb 2, as shown in FIG. 2. In the halogen lamp, the halogen element sealed in the bulb 2, for example iodine I, is combined with the tungsten W vaporized from the filament 3 to form tungsten halide WI.sub.2. The tungsten halide WI.sub.2 is decomposed into tungsten W and iodine I as the filament 3 is heated. Consequently, the tungsten W returns to the filament 3. This phenomenon is called a halogen cycle. Namely, in a halogen lamp, the light intensity is maintained stable for a long period of time by the repetition of the halogen cycle.
By contrast, in an ordinary incandescent lamp, the halogen cycle does not occur even if tungsten W is deposited on the wall 2a of the bulb 2. Consequently, the wall of the bulb becomes black and the intensity is gradually reduced with time.
The halogen lamp has the above described advantage. However, the halogen lamp was employed as a light source in a contact printing machine for daylight condition. The reason will be described in the following. The emission light spectrum of the halogen lamp includes the photosensitive wavelength range (about 400 nm) of the photosensitive material for daylight condition to which yellow light is the safe light. Therefore, the halogen lamp inherently has the intensity which can be used as the light source for a contact printing machine for daylight condition. However, on-time of an exposure apparatus is relatively short (2 to 10 seconds) which is not long enough for the circulation of the halogen cycle. Tungsten W is deposited at a portion of the bulb where the temperature is the lowest, turning the wall of the bulb black and accordingly, causing reduction of intensity.
Therefore, a metal halide lamp or the like has been used as the light source. A cooling apparatus, a spare lighting circuit, an explosion proof interlock circuit and the like must be provided when a metal halide lamp is used. Consequently, the light source apparatus for exposure as a whole becomes expensive.